This invention relates to particulate semiconductor devices, particularly high efficiency photovoltaic devices.
Although considerable progress has been made in the cost-effective manufacture of large area devices, particularly large area amorphous silicon devices, the efficiencies of mass produced solar panels are in need of improvement. While laboratory schemes can produce small area amorphous silicon devices with efficiencies over 10 %, it is difficult to raise the mass production efficiency to over 7-10% with a single junction structure.
Accordingly, it is an object of the invention to develop low cost solar cells for mass produced panels with efficiencies of well over 10%.
One approach has been to combine, in tandem, an amorphous silicon top cell and a crystalline silicon bottom cell. Such a configuration has not been realized to date at a reasonably low cost. Another approach is to use amorphous silicon and amorphous silicon alloys. This does not provide a good match of bandgaps. The reason is that the bandgap of amorphous silicon alloys cannot be changed within the necessary range without seriously degrading the electrical properties of the absorbing layer.
Accordingly, it is a further object of the invention to develop low cost crystalline silicon devices. Such devices are inherently stable.
One approach to cost reduction for crystalline silicon is the production of solar cells from cast polycrystalline silicon ignots. This process has the advantage that no single crystals need to be grown, a step which is time and energy consuming. Although the remaining process steps are essentially those used in the manufacture of single crystalline solar cells, costs are reduced by a factor of more than two and energy amortization time is reduced from ten years to approximately one year.
Another approach is to use silicon ribbon technology where wafers need not be cut from an ingot. Thus processing time and material loss are reduced. These advantages are offset by numerous manufacturing problems associated with handling and processing liquid silicon (at 1414 degrees Centigrade), and the growing of polycrystalline film.
A further attempt to realize low cost crystalline thin film cells involves temperature gradient liquid phase epitaxy out of a silicon-tin solution. The growth temperatures range from 800-1000 degrees Centigrade and thus are markedly lower than the melting point of silicon. Stainless steel is used as a substrate. Efficiencies close to 10% have been reported for small area devices. However, in this method there is a lattice mismatch between the substrate and the grown layer, which inhibits continuous film growth and ultimately reduces efficiency. Furthermore the growth temperature is still very high, confining the choice of materials that are suitable as substrates.
A different approach involves the use of silicon particles. A slurry consisting of silicon particles, water, and polyvinylalcohol is spread on a suitable heat-resistant substrate. Subsequently the particulate layer is remelted. Efficiencies of 5% have been reported, but high-cost substrates are required.
Another approach is by fusing silicon particles with a pulsed electric current. No useful devices have been reported to date.
A further approach is to sinter silicon particles to an aluminum coated, stainless steel substrate. The particulate layer is covered with an insulating material, e.g. a resin or a low temperature glass, in order to fill voids and to seal the rear contact. The top surface is then ground or etched to expose bare silicon. Finally a junction is formed by ion implantation, epitaxial deposition, or diffusion. While ion implantation appears feasible, epitaxial deposition and diffusion are questionable. The particulate layer is not capable of withstanding temperatures higher than the melting or flaming point of the sealant or the melting point of the aluminum.
The foregoing particulate devices use crushed or milled particles prepared from a single or polycrystalline ingot. Usually the particles are etched after milling in order to remove surface contamination. Silicon ball mills and silicon lumps as grinding media are used to minimize contamination.